client.go

package arp

import (
	"errors"
	"net"
	"net/netip"
	"time"

	"github.com/mdlayher/ethernet"
	"github.com/mdlayher/packet"
)

// errNoIPv4Addr is returned when an interface does not have an IPv4
// address.
var errNoIPv4Addr = errors.New("no IPv4 address available for interface")

// protocolARP is the uint16 EtherType representation of ARP (Address
// Resolution Protocol, RFC 826).
const protocolARP = 0x0806

// A Client is an ARP client, which can be used to send and receive
// ARP packets.
type Client struct {
	ifi *net.Interface
	ip  netip.Addr
	p   net.PacketConn
}

// Dial creates a new Client using the specified network interface.
// Dial retrieves the IPv4 address of the interface and binds a raw socket
// to send and receive ARP packets.
func Dial(ifi *net.Interface) (*Client, error) {
	// Open raw socket to send and receive ARP packets using ethernet frames
	// we build ourselves.
	p, err := packet.Listen(ifi, packet.Raw, protocolARP, nil)
	if err != nil {
		return nil, err
	}
	return New(ifi, p)
}

// New creates a new Client using the specified network interface
// and net.PacketConn. This allows the caller to define exactly how they bind to the
// net.PacketConn. This is most useful to define what protocol to pass to socket(7).
//
// In most cases, callers would be better off calling Dial.
func New(ifi *net.Interface, p net.PacketConn) (*Client, error) {
	// Check for usable IPv4 addresses for the Client
	addrs, err := ifi.Addrs()
	if err != nil {
		return nil, err
	}

	ipaddrs := make([]netip.Addr, len(addrs))
	for i, a := range addrs {
		ipPrefix, err := netip.ParsePrefix(a.String())
		if err != nil {
			return nil, err
		}
		ipaddrs[i] = ipPrefix.Addr()
	}

	return newClient(ifi, p, ipaddrs)
}

// newClient is the internal, generic implementation of newClient.  It is used
// to allow an arbitrary net.PacketConn to be used in a Client, so testing
// is easier to accomplish.
func newClient(ifi *net.Interface, p net.PacketConn, addrs []netip.Addr) (*Client, error) {
	return &Client{
		ifi: ifi,
		ip:  firstIPv4Addr(addrs),
		p:   p,
	}, nil
}

// Close closes the Client's raw socket and stops sending and receiving
// ARP packets.
func (c *Client) Close() error {
	return c.p.Close()
}

// Request sends an ARP request, asking for the hardware address
// associated with an IPv4 address. The response, if any, can be read
// with the Read method.
//
// Unlike Resolve, which provides an easier interface for getting the
// hardware address, Request allows sending many requests in a row,
// retrieving the responses afterwards.
func (c *Client) Request(ip netip.Addr) error {
	if !c.ip.IsValid() {
		return errNoIPv4Addr
	}

	// Create ARP packet for broadcast address to attempt to find the
	// hardware address of the input IP address
	arp, err := NewPacket(OperationRequest, c.ifi.HardwareAddr, c.ip, ethernet.Broadcast, ip)
	if err != nil {
		return err
	}
	return c.WriteTo(arp, ethernet.Broadcast)
}

// Resolve performs an ARP request, attempting to retrieve the
// hardware address of a machine using its IPv4 address. Resolve must not
// be used concurrently with Read. If you're using Read (usually in a
// loop), you need to use Request instead. Resolve may read more than
// one message if it receives messages unrelated to the request.
func (c *Client) Resolve(ip netip.Addr) (net.HardwareAddr, error) {
	err := c.Request(ip)
	if err != nil {
		return nil, err
	}

	// Loop and wait for replies
	for {
		arp, _, err := c.Read()
		if err != nil {
			return nil, err
		}

		if arp.Operation != OperationReply || arp.SenderIP != ip {
			continue
		}

		return arp.SenderHardwareAddr, nil
	}
}

// Read reads a single ARP packet and returns it, together with its
// ethernet frame.
func (c *Client) Read() (*Packet, *ethernet.Frame, error) {
	buf := make([]byte, 128)
	for {
		n, _, err := c.p.ReadFrom(buf)
		if err != nil {
			return nil, nil, err
		}

		p, eth, err := parsePacket(buf[:n])
		if err != nil {
			if err == errInvalidARPPacket {
				continue
			}
			return nil, nil, err
		}
		return p, eth, nil
	}
}

// WriteTo writes a single ARP packet to addr. Note that addr should,
// but doesn't have to, match the target hardware address of the ARP
// packet.
func (c *Client) WriteTo(p *Packet, addr net.HardwareAddr) error {
	pb, err := p.MarshalBinary()
	if err != nil {
		return err
	}

	f := &ethernet.Frame{
		Destination: addr,
		Source:      p.SenderHardwareAddr,
		EtherType:   ethernet.EtherTypeARP,
		Payload:     pb,
	}

	fb, err := f.MarshalBinary()
	if err != nil {
		return err
	}

	_, err = c.p.WriteTo(fb, &packet.Addr{HardwareAddr: addr})
	return err
}

// Reply constructs and sends a reply to an ARP request. On the ARP
// layer, it will be addressed to the sender address of the packet. On
// the ethernet layer, it will be sent to the actual remote address
// from which the request was received.
//
// For more fine-grained control, use WriteTo to write a custom
// response.
func (c *Client) Reply(req *Packet, hwAddr net.HardwareAddr, ip netip.Addr) error {
	p, err := NewPacket(OperationReply, hwAddr, ip, req.SenderHardwareAddr, req.SenderIP)
	if err != nil {
		return err
	}
	return c.WriteTo(p, req.SenderHardwareAddr)
}

// Copyright (c) 2012 The Go Authors. All rights reserved.
// Source code in this file is based on src/net/interface_linux.go,
// from the Go standard library.  The Go license can be found here:
// https://golang.org/LICENSE.

// Documentation taken from net.PacketConn interface.  Thanks:
// http://golang.org/pkg/net/#PacketConn.

// SetDeadline sets the read and write deadlines associated with the
// connection.
func (c *Client) SetDeadline(t time.Time) error {
	return c.p.SetDeadline(t)
}

// SetReadDeadline sets the deadline for future raw socket read calls.
// If the deadline is reached, a raw socket read will fail with a timeout
// (see type net.Error) instead of blocking.
// A zero value for t means a raw socket read will not time out.
func (c *Client) SetReadDeadline(t time.Time) error {
	return c.p.SetReadDeadline(t)
}

// SetWriteDeadline sets the deadline for future raw socket write calls.
// If the deadline is reached, a raw socket write will fail with a timeout
// (see type net.Error) instead of blocking.
// A zero value for t means a raw socket write will not time out.
// Even if a write times out, it may return n > 0, indicating that
// some of the data was successfully written.
func (c *Client) SetWriteDeadline(t time.Time) error {
	return c.p.SetWriteDeadline(t)
}

// HardwareAddr fetches the hardware address for the interface associated
// with the connection.
func (c Client) HardwareAddr() net.HardwareAddr {
	return c.ifi.HardwareAddr
}

// firstIPv4Addr attempts to retrieve the first detected IPv4 address from an
// input slice of network addresses.
func firstIPv4Addr(addrs []netip.Addr) netip.Addr {
	for _, a := range addrs {
		if a.Is4() {
			return a
		}
	}
	return netip.AddrFrom4([4]byte{0, 0, 0, 0})
}